The present systems and methods relate to fuel cell based power generation systems for the generation of electrical energy, and particularly, to improving the freeze-start capability of such systems.
Fuel cells are electrochemical energy converters, which directly convert chemical energy to electrical energy. For this purpose, the fuel cell is supplied with a fuel and with oxidant (such as air) as reactants. The reactants are spatially separated by an electrolyte, through which ion exchange takes place. Example fuels include hydrogen, or methane. There are several known types of fuel cells, including polymer electrolyte membrane fuel cells (PEMFC), phosphoric acid fuel cells (PAFC), and solid oxide fuel cells (SOFC). This list is not complete and the present systems and methods are not limited to a specific type of fuel cell, nor to a specific fuel, nor to a specific application. For example, the application of the present systems and methods invention in a motor vehicle is only one possible embodiment.
Water is present in fuel cells. It may be produced in the fuel cell as a product of the reaction of hydrogen and oxygen, or it may be supplied to the fuel cell for cooling or humidification purposes. For example, the membrane serving as electrolyte in PEM fuel cells must be humidified to allow an efficient cation exchange. Thus, typically either or both of the fuel and oxidant streams may be humidified with water in a humidifier upstream of the fuel cell. Water may also be conducted through special cooling channels in the fuel cell to cool it.
After an operational shutdown of a fuel cell, the temperature of the fluids contained therein and of the components of the power generation system gradually drop to ambient temperature. During this period, water vapor that is still present in the fluid channels of the power generation system condenses and precipitates as liquid water. If the ambient temperature drops below 0° C., any water present in the power generation system may freeze. Such water may be located in the fuel cells, but may also be present in other areas of the power generation system, such as circulation devices (e.g., pumps, compressors, fans, blowers) for the reactants, valves, or in the flow channels that conduct the reactant streams or the cooling water through the power generation system. Often, the flow channels have areas in which water can accumulate, such as in corners or at the end of dead ends where sensors are located.
Problems may occur upon resumption of power generation if condensed water drops or ice are still present in the system. The presence of ice or condensed water may obstruct the flow of reactants, and the presence of ice in particular may affect the proper functioning of system components, such as valves, sensors, or circulation devices. In some situations, this may result in damage to the components.
To prevent accumulation of water drops and ice and to improve the freeze-starting capability of fuel cell systems, a conventional approach is to purge (i.e., blow dry gas through) the flow channels of the system immediately after operation ceases. However, this method has disadvantages. The use of purging requires considerable amounts of time and energy. Moreover, as the quantity of water present in the system is unknown, it is difficult to estimate whether the amount of purge gas and the duration of the purging will be adequate for sufficient drying. Furthermore, it is difficult for the purge gas to reach water that has been deposited at poorly accessible spots of the flow channel system, such as at the ends of flow channels and in corners. Moreover, the membranes of PEM fuel cells can normally not be dried completely. There will always be small remaining reservoirs, from which water is able to diffuse to other locations and, in particular, to critical positions in the power generation system.
Japanese patent document JP 2003-142136 proposes the provision of a condenser for drying the internal fluid channels of a fuel cell stack, where the condenser is cooled during a power generating operation. For vehicular fuel cell stacks, Japanese patent document JP 2003-142136 proposes disposing the condenser just behind the radiator grill, so that it is cooled by the air draft. The object of this arrangement is to permit water vapor that is still present in the fluid channels of the fuel cell stack after operation ceases to travel through an open path to the condenser, and to precipitate there. Thus, the condenser forms a predetermined condensation point.
An alternate approach to minimize or avoid the problems associated with freezing water in a fuel cell stack is to house the fuel cell stack in a thermally insulated container. This approach is described in published U.S. Patent Application No. 2003/0162063. One drawback of using the thermally insulated container housing only the fuel cell stack, however, is that the other components of the power generation system may retain water in inaccessible locations. The system described in US 2003/0162063 keeps residual water in the fuel cell stack from freezing by maintaining the temperature in the insulating container sufficiently above freezing with a heating system arranged in the insulating container. However, water droplets may remain in the reactant channels of the power generation system after an operational shutdown.